The present invention relates to a process for leaching gold and silver from noble metal containing materials selected from ores, ore concentrates or wastes of precious metals, in particular from oxide and sulfide ores or ore concentrates, also from waste materials of earlier incomplete leaching, using an aqueous cyanide leaching solution with a pH value of 8 to 13 while adding hydrogen peroxide to the ore slurry of an agitated leach or to the barren solution of heap leach and while maintaining an oxygen concentration of 2 to 20 mg/liter in the leaching solution. By adding decomposition catalysts for hydrogen peroxide and where called for separating the formed gold- and silver-cyano-complexes from the leach solution during leaching, the demand for chemicals can be lowered and higher yield of precious metals may be achieved in less leaching time. The process of the invention has made it possible to substantially increase the economy of leaching while using hydrogen peroxide as the oxidant.
It is known to convert gold and silver by means of a cyanide aqueous solution in the presence of atmospheric oxygen into soluble cyano-complexes. This is the principle used to leach these precious metals from ores or ore concentrates or from wastes of precious metals, for instance electronics scrap. In so-called agitation leaching, the finely ground ore is dispersed in a water slurry and following the pH adjustment and addition of an aqueous cyanide solution with air gassing in cascaded, cylindrical agitation vessels or Pachuca tanks, it will then be agitated up to 48 hours and leached during the procedure. Hereafter the aqueous phase of the ore slurry shall be called the leach solution. In industry, besides the agitation leach solution, use is also made of so-called heap leaching, whereby a leach solution as a rule adjusted between a pH of 9 and 12 and containing cyanide is made to drip onto heaps of ore, atmospheric oxygen being the oxidant.
Both in heap leaching and in agitation leaching, an oxygen deficiency may arise in the ore slurry, resulting in limited gold yield and lowered rate of leaching. Attempts already have been undertaken to leach precious metal ores in the presence of oxidants other than atmospheric air. Among these approaches is the use of hydrogen peroxide.
Illustratively U.S. Pat. No. 732,605 discloses a procedure for leaching ores, wherein the ore is mixed with a metal oxide and this mixture is treated in a cyanide solution containing the hydrogen peroxide that reduces the metal oxide. The quantity of metal oxide that can be reduced, such as Fe.sub.2 O.sub.3, MnO.sub.2, Ag.sub.2 O and HgO, should be at least equivalent to the quantity of hydrogen peroxide. The more of the metal oxide is present, the more advantageous the procedure in relation to forming nascent oxygen. Prior to leaching, the ore must be so carefully mixed with such a quantity of metal oxide that there is a content of at least 0.05%, i.e., 500 ppm. This procedure has been known since 1903 and manifestly applies only to heap leaching. It is not used today in industry because of failing to meet the requirements of commercial practice. Adding a metal oxide in the above proportions not only is uneconomical because of costs and of the time of required mixing, but also increased consumption of cyanide must be expected because the metals involved in part themselves will form cyano-complexes. Because of the presence of the admixed metal oxides in the required amounts, leaching malfunctions, for instance by passivation, can occur. Also final waste treatments to detoxify the liquids from cyanides are inevitable.
The leaching procedure known from the U.S. Pat. No. 3,826,723 using a cyanide leaching solution and hydrogen peroxide in addition to the presence of an alkali cyanide and a stabilized hydrogen peroxide solution furthermore requires a lignin sulfonate. The large quantities of chemical admixtures, especially of a NACN and H.sub.2 O.sub.2, stated in this document, render an economical procedure not feasible.
The procedure of the Canadian Patent 1,221,842 is for leaching for instance gold and silver from pyrite concentrates, the leaching solution containing an alkali cyanide, an alkali carbonate and hydrogen peroxide. In the preferred mode of implementation, the leaching solution contains at least 2% by weight of sodium cyanide and at least 2% by weight of sodium carbonate and 0.6% by weight hydrogen peroxide. At these high concentrations, sodium cyanide is oxidized to a substantial extent by hydrogen peroxide according to the known cyanide detoxification using hydrogen peroxide thereby entailing additional consumption of these chemicals. In order to be able to feed back the cyanide leaching solution following the separation of the precious metal cyano-complexes into the leaching cycle, expensive purification is needed. In view alone of the high demand for chemicals, this procedure is uneconomical.
As shown by the German Patent 36 37 082, the leaching procedure employing cyanide and hydrogen peroxide may be more economical by so controlling the addition of the aqueous H.sub.2 O.sub.2 solution by means of the oxygen concentration in the leaching solution that this leaching solution contains between 2 and 20 mg, preferably 7 to 13 mg of O.sub.2 /liter Accordingly in this known procedure, the hydrogen peroxide is added to the leaching solution or ore slurry not in one batch, but continuously or periodically in measured amounts. The lowest rates of consumption of hydrogen peroxide apply when this hydrogen peroxide is supplied as a very dilute aqueous solution, preferably with a content of 0.5 to 5, in particular 1 to 2% by weight.
However, where the procedure of the German Patent 36 37 082 has been applied in practice in gold mines, it was found that the need for sodium cyanide and hydrogen peroxide does not always meet expectations in spite of carefully metering the H.sub.2 O.sub.2 and using a diluted H.sub.2 O.sub.2 solution. In some cases, for instance when leaching pyritic ore concentrates and/or waste materials from earlier, incomplete leachings, the demand for chemicals was unsatisfactorily high. Manifestly this increased demand is related to the physical and chemical properties of the ores being leached.
Accordingly it is the object of the present invention to so improve the procedure of the German Patent 36 37 082 that the demand for hydrogen peroxide and where possible also that for sodium cyanide is lowered. It is furthermore desired that the demand for chemicals be made more independent of the compositions and natural variations of the ores to be leached. In addition it is highly significant to the mining industry to recover the maximum amount of precious metal which can be leached with cyanide in a shorter time than heretofore.